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Category: Technology

Dockerizing a Rails Application

Adding docker to a Ruby on Rails application might seem like a herculean task at first, but it’s actually not as difficult as it seems. In this article, I will walk you through Docker and how to dockerize a rails application.

What is Docker?

Docker is a tool designed to make it easier to create, deploy, and run applications by using containers. Containers allow a developer to package up an application with all of the parts it needs, such as libraries and other dependencies, and deploy it as one package. Docker is a set of platform-as-a-service products that use OS-level virtualization to deliver software in packages called containers.

To install docker on your computer, click here and select your OS.

Creating a Rails Application

I am using rails 6 to create a new rails application called docker-rails, with Postgres and without running a test.

rails new docker-rails -T --database=postgresql

Dockerizing the Application

Add a file called Dockerfile to the root directory of your application. Notice the file has no extension. Add the following lines of code in that file. I will explain each line later.

  1. FROM ruby:2.7.1
  2. RUN curl -sL | bash – \ && apt-get install -y nodejs postgresql-client
  3. RUN mkdir /docker-rails
  4. WORKDIR /docker-rails
  5. COPY Gemfile /docker-rails/Gemfile
  6. COPY Gemfile.lock /docker-rails/Gemfile.lock
  7. RUN bundle install –full-index
  8. COPY . /docker-rails
  9. COPY /usr/bin/
  10. RUN chmod +x /usr/bin/
  11. ENTRYPOINT [“”]
  12. EXPOSE 3000
  13. CMD [“rails”, “server”, “-b”, “”]

 On line 1 we download Ruby; version 2.7.2 in this case.

 On line 2 we install NodeJS along with all its dependencies and Postgres.

 The code on line 3 creates a directory I called docker-rails. But you can call yours any name you like.

 Line 4 sets docker-railsas our working directory that holds all our files.

 Line 5 copies the content of the Gemfile from our directory to docker-rails/Gemfile in the container.

 Line 6 also copies the content of Gemfile.lock.

 Line 7 runs bundle install to install and update our new Gemfile and Gemfile.lock.

 Line 8 copies the entire content from the current directory to our working directory in the container docker-rails/.

 Line 9 copies the content of a file named We will add the file in a bit.

 Line 10 grants access to the file while line number 11 specifies the file to be used as an entrypoint.

 Line 12 exposes port 3000 so we can run the rails app on our browser on port 3000.

 Finally, Line 13 contains the command that will be executed to start or run the container.

Add a file named and copy the following to it.

set -e 

# If the database exists, migrate. Otherwise setup #(create and migrate) 

bundle exec rake db:migrate 2>/dev/null || bundle exec 

rake db:create db:migrate echo "Database has been created & migrated!"

# Remove if it exists 

rm -f /docker-rails/tmp/pids/

# execute container's main process (CMD in docker file) exec "$@"

The above code is executed when the entrypoint script is called in the Dockerfile. Add a file named docker-compose.yml. It will contain dependencies of the application. The code that should be in the file is shown below:

version: '3' 
    image: postgres 
            POSTGRES_PASSWORD: 'password1' 
            PGHOST: "db" 
            PGUSER: "postgres" 
            PGDBNAME: "docker-rails" 
            - ./tmp/db:/var/lib/postgresql/data 
        build: . 
            command: bash -c "rm -f tmp/pids/ && bundle exec rails s -p 3000 -b ''" 
        - .:/docker-rails 
        ports: - "3000:3000" 
            - db 

To build our docker container we run docker-compose build in the project root directory. This will download all required dependencies. After that, run docker-compose up to start our docker container. Visit localhost:3000 to see your app running in a docker container.

Phew! You just dockerized your first rails application, not so difficult…was it?